Versatile audio power amplifier

ABSTRACT

An audio power amplifier includes a first and a second amplification unit, each including a switching voltage amplifier, an output filter, a current compensator, an inner current feedback loop feeding a measurement of current measured at the output inductor back to a summing input of the current compensator, a voltage compensator coupled to the summing input of the current compensator, and an outer voltage feedback loop. A controlled signal path provides the output of the voltage compensator of the first amplification unit to the current compensator of the second amplification unit. The first and second amplification units are operable with separate loads, in parallel driving a common load, or across a bridge-tied-load. A second pair of amplification units may be added and operated together with the first pair to drive a single speaker with a parallel pair of amplifiers on each side of a bridge-tied-load.

This application is a continuation of U.S. patent application Ser. No.14/021,434 filed on Sep. 9, 2013 and titled “Versatile Audio PowerAmplifier,” which is a continuation of U.S. patent application Ser. No.13/160,806 filed on Jun. 15, 2011 and titled “Versatile Audio PowerAmplifier” (now U.S. Pat. No. 8,558,618), which is a continuation ofU.S. patent application Ser. No. 12/717,198 filed on Mar. 4, 2010 andtitled “Versatile Audio Power Amplifier” (now U.S. Pat. No. 7,986,187),each of which is incorporated by reference in its entirety.

BACKGROUND

This disclosure relates to a versatile audio power amplifier.

Switching amplifiers, also called Class D amplifiers, amplify an inputsignal by modulating that signal into a series of pulses that drive acomplementary pair of transistors operated in the switching mode. Thetransistors alternately couple positive and negative power sources tothe output, which in total produce an amplified representation of theinput signal.

SUMMARY

In general, in some aspects, an audio power amplifier includes a firstand a second amplification unit. Each amplification unit includes aswitching voltage amplifier having a command signal input and anamplified signal output, an output filter between the amplified signaloutput and a load terminal, a current compensator with acurrent-compensated command signal output coupled to the command signalinput of the voltage amplifier, an inner current feedback loop feeding ameasurement of current measured at the output inductor back to a summinginput of the current compensator, a voltage compensator with avoltage-compensated control signal output coupled to the summing inputof the current compensator, and an outer voltage feedback loop feedingvoltage at the load terminal back to a summing input of the voltagecompensator. A first controlled signal path provides thevoltage-compensated control signal output of the voltage compensator ofthe first amplification unit to the summing input of the currentcompensator of the second amplification unit. The second amplificationunit uses the voltage-compensated control signal of the firstamplification unit as input to the current compensator of the secondamplification unit in place of the voltage-compensated command signal ofthe second amplification unit when the first controlled signal path isactivated. Control electronics provide signal inputs to the first andsecond amplification units and control the first controlled signal pathsuch that the first and second amplification units are operable withseparate loads, in parallel driving a common load, or across abridge-tied-load.

Implementations may include one or more of the following features. Thefirst and second amplification units may be operable with separate loadsby each amplifying separate signals and providing the amplified signalson their separate output terminals. The first and second amplificationunits may be operable in parallel driving a common load by eachamplifying the same signal, provided from the first amplification unitto the second amplification unit via the first controlled signal path,and providing identical amplified signals on their separate outputterminals, which are to be coupled to a common input terminal of theload. The first and second amplification units may be operable across abridge-tied-load by amplifying a first signal in the first amplificationunit and amplifying an inverted copy of the first signal in the secondamplification unit, and providing their respective amplified signals ontheir separate output terminals, which are to be coupled to separateinput terminals of the load.

A third and a fourth amplification unit identical to the first andsecond amplification units, and a second controlled signal path from thevoltage-compensated control signal output of the voltage compensator ofthe third amplification unit to the summing input of the currentcompensator of the fourth amplification unit may be included, thecontrol electronics further providing signal inputs to the third andfourth amplification units and controlling the second controlled signalpath such that the third and fourth amplification units are operablewith separate loads, in parallel driving a common load, or across abridge-tied-load, and all four of the amplification units are operabletogether with the first and second units in parallel driving a firstside of a bridge-tied-load, and the third and fourth units in paralleldriving a second side of the bridge-tied-load.

The four amplification units may be operable together by amplifying afirst signal in each of the first and second amplification units,provided from the first amplification unit to the second amplificationunit via the first controlled signal path, and providing identicalamplified first signals on the separate output terminals of the firstamplification unit and the second amplification unit, amplifying aninverted copy of the first signal in each of the third and fourthamplification units, provided from the third amplification unit to thefourth amplification unit via the second controlled signal path, andproviding identical amplified inverted first signals on the separateoutput terminals of the third amplification unit and the fourthamplification unit, the output terminals of the first and secondamplification units are to be coupled to a first input of the load, andthe output terminals of the third and fourth amplification units are tobe coupled to a second input of the load.

The amplifier may use a four-quadrant power supply having a synchronousoutput rectifier. The synchronous output rectifier may include a MOSFET.The first controlled signal path may include a switch controlled by thecontrol electronics. The switching voltage amplifiers may each include amodulator, a gate driver, a pair of transistors, and a pair of diodescoupled between the source and drain terminals of the transistors. Thetransistors may include MOSFETS, the diodes being intrinsic to theMOSFETS. The output filter may include an output inductor and themeasured current may be the current through the output inductor.

In general, in some aspects, amplifying audio-frequency signalsincludes, in each of a first and a second amplification unit, amplifyinga current-compensated command signal in a switching voltage amplifier toprovide an amplified signal output, measuring current through an outputfilter between the amplified signal output of the voltage amplifier anda load terminal to produce a current measurement, feeding back thecurrent measurement to a summing input of a current compensator via aninner current feedback loop, at the current compensator, comparing thecurrent measurement to a voltage-compensated command signal andproviding the current-compensated command signal to the voltageamplifier, feeding back voltage at a load terminal of the amplificationunit to a summing input of a voltage compensator via an outer voltagefeedback loop, and at the voltage compensator, comparing the feedbackvoltage to an input command signal and providing the voltage-compensatedcommand signal to the summing input of the current compensator. A firstcontrolled signal path from an output of the voltage compensator of thefirst amplification unit to the summing input of the current compensatorof the second amplification unit is controlled to selectively providethe voltage-compensated command signal of the first amplification unitto the summing input of the second amplification unit in place of thevoltage-compensated command signal of the second amplification unit.Signal inputs are provided to the first and second amplification unitsand the first controlled signal path is controlled to selectivelyoperate the first and second amplification units with separate loads, inparallel driving a common load, or across a bridge-tied-load.

Advantages include comprehensive configurability with high efficiency.The amplifier can serve a wide variety of connection topologies, loadimpedances, and power levels without hardware modification. Being ableto drive loudspeakers at a wide range of impedances from a singleamplifier allows the amplifier to support a diverse range of audiosystem configurations without requiring a diverse set of amplifierproducts.

Other features and advantages will be apparent from the description andthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1D show block diagrams of amplifier-speaker topologies.

FIG. 2 shows a circuit diagram of a single amplifier stage.

FIG. 3 shows a circuit diagram of two amplifier stages in a bridgedconfiguration.

FIG. 4 shows a circuit diagram of two amplifier stages in a parallelconfiguration.

FIG. 5 shows a circuit diagram of four amplifier stages in aparallel-bridged configuration.

FIG. 6 shows a circuit diagram of a configurable amplifier system.

FIG. 7 shows a circuit diagram of an isolation converter.

FIGS. 8A through 8C show power flow through the isolation converter ofFIG. 7.

FIG. 9A shows a circuit diagram of a representative class D amplifier.

FIGS. 9B and 9C show energy flow through the amplifier of FIG. 9A.

DESCRIPTION

Power amplifiers may be connected to their loads in a number oftopologies, depending on the nature and intended use of the loads. FIGS.1A through 1D show four topologies for connecting power amplifiers toloudspeakers. In FIG. 1A, a single amplifier 10 drives a loudspeaker 20by providing power to one terminal of the loudspeaker, while the otherterminal is grounded. This is a typical configuration sometimes referredto as a “half-bridge.” In FIG. 1B, a “full-bridge” configuration isshown, where two amplifiers 10 and 12 are used, one connected to eachterminal of the loudspeaker 22. The second amplifier 12 is driven withan inverse of the signal to the first amplifier 10, so the total voltageacross the loudspeaker 22 is doubled, while the current remains the sameas that in the half-bridge configuration. By providing twice the voltageat the same current, this configuration can drive loudspeakers withlarger impedances than can be driven by the half-bridge. It can providemore power total, or the same power with less dissipation per amplifier.This mode of operation is ubiquitous in the audio amplifier field, andis often referred to as ‘BTU’ (bridge tied load) configuration.

In FIG. 1C, the two amplifiers 10 and 12 are connected in parallel to acommon terminal on the loudspeaker 24, while the other terminal isgrounded. This is referred to as a “parallel” configuration. Theparallel configuration delivers twice the current at the same voltage asthe half-bridge configuration, which is useful for driving smallerimpedances at the same power level as the BTL configuration. Forexample, if the half-bridge is optimized to provide 500 W to a 4Ω load,the current required to provide the same power into a 2Ω load or thevoltage required to provide the same power to an 8Ω load may be at orbeyond the limits of the amplifier. The parallel configuration can drive500 W into a 2Ω load with half the dissipation of the singlehalf-bridge, or drive a full 1 kW if each half-bridge can handle thecurrent. The BTL configuration, on the other hand, can drive 500 W intoan 8Ω load without approaching its voltage limits, or drive a full 1 kWif the voltages are available.

Finally, in FIG. 1D, four amplifiers 10, 12, 14, and 16 are used, with aparallel pair of amplifiers connected to each terminal of theloudspeaker 26. One pair, 10 and 12, is driven with the opposite signalof the other pair, 14 and 16. This is referred to as a parallel-bridgedconfiguration, and delivers twice the voltage and twice the current as asingle half-bridge, for four times the power. Using the same example asabove, if each half-bridge is optimized for 500 W at 4Ω, theparallel-bridged configuration can deliver 2 kW to a 4Ω load, with thesame voltage and current per amplifier stage.

With a class D amplifier, management of current-sharing between twoparallel amplifiers is more important than in linear, or class AB,amplifiers, because the dissipated power in a switching device isproportional to I², rather than to I, as it is in a linear amplifier.Sharing current between two identical devices will decrease conductionloss by roughly a factor of two in the switching amplifier. The decreasewill actually be a bit more than a factor of two because there arefurther gains due to the temperature coefficient of resistance of theFET—decreasing the current also decreases the temperature, which in turndecreases the intrinsic resistance of the device. If the current is notcontrolled, however, it is likely that one of the two devices willdeliver substantially more current than the other, losing the benefitsof parallel operation and possibly damaging the amplifiers.

To provide efficient current sharing in topologies, a feedback loop maybe added to the amplifier. For example, as shown in FIG. 2, a unit cell100 provides one half-bridge class D audio amplifier, shown connected toan arbitrary loudspeaker 120. The core of the amplifier includes amodulator and gate driver 102, a switching power-output stage made up oftransistors 104 and 106, and an output filter including an inductor 108and capacitor 110. The control system includes an outer voltage loop 112that feeds back the output voltage at the load 120 to a summer 114 andvoltage-loop compensator 116. The summer 114 also receives the inputvoltage command V_(c-in). The modulator and gate driver 102, incombination with the transistors 104 and 106, constitute a voltageamplifier.

To allow current sharing when two of these unit cells 100 are connectedin parallel, an inner current loop 130 is provided. The inner currentloop 130 feeds back a measurement of output current, from a currentsense 130 a, to another summer 132 and current-loop compensator 134. Theinner current loop controls the output current of the amplifier, so thattwo amplifiers operating in parallel will each provide half the totalcurrent; neither will attempt to deliver all the current and lose theadvantages of parallel operation. In this configuration, the currentloop around the core voltage amplifier turns the system into a currentamplifier, and the outer voltage loop turns the entire unit cell 100back into a voltage amplifier.

The inner current loop provides some additional advantages. The currentloop naturally provides current limiting within the unit cell. That is,the feedback 130 the current-loop compensator 134 prevents the commandinto the modulator 102 from causing a current gain in excess of themaximum current command from the voltage-loop compensator 116, even ifthe load is shorted. Additionally, because the inner current loopprovides control, the voltage measurement used for the outer voltageloop can be moved outside the output filter, closer to the load (asshown). (An output filter typically imposes a 180° phase shift, aroundwhich a control loop could not be closed.) Moving the voltage loop toafter the output filter allows the amplifier to support a greatervariety of loads while the inner current loop maintains stability. Theinner current loop 130 can also be used to provide pulse width errorcorrection in the modulator 102, as described in U.S. patent applicationSer. No. 12/717,224, titled “Reducing Pulse Error Distortion,” and filedthe same day as this application, the entire contents of which areincorporated here by reference.

The summers 114 and 132 are not necessarily discrete components, but maybe, for example, summing inputs of the compensators 116 and 134. Thecompensators preferably are built from standard circuit components,i.e., op-amps and associated circuitry. The current-sensing element 130a may be any standard current-sensing technology, such as discreteHall-effect sensors. The output inductor 108 may be formed using planarwindings on a printed circuit board, and part of the current sense isprovided by a current sense winding integrated into the output inductor108, as described in U.S. patent application Ser. No. 12/717,208, titled“Planar Amplifier Output Inductor with Current Sense,” and filed thesame day as this application, the entire contents of which areincorporated here by reference. As explained in that patent application,forming the current-sense winding as part of the PCB windings of theoutput inductor advantageously shields the current-sense signal fromnoise within the inductor. The inductor's current-sensing windingindicates an AC component of the current, while a Hall-effect sensor mayalso be used to indicate the DC component of the current.

In some examples, these unit cells are combined in groups of 4. Eachgroup has appropriate interconnections within the control system,allowing a single set of four unit cells to provide any of thetopologies shown in FIGS. 1A through 1D. The user, through controlsoftware, for example, may specify the particular topology needed. Todetect connection problems and confirm that the topology of connectedloudspeakers matches the configuration of the amplifier components, anamplifier product may include circuitry for detecting the type andtopology of the connected loudspeakers. One such system is described inU.S. patent application Ser. No. 12/114,265, titled “Detecting aLoudspeaker Configuration,” filed May 2, 2008, the entire contents ofwhich are incorporated here by reference. Such a system may also be usedto discover the topology of the connected loudspeakers and automaticallyconfigure the amplifier accordingly.

For independent half-bridge operation of multiple channels, each unitcell is connected to one loudspeaker as shown in FIG. 2, and separatesignals are provided to each unit cell.

As shown in FIG. 3, two half bridges 100 a and 100 b may be combinedinto a full bridge configured to drive a single loudspeaker 120 b as abridge-tied-load to provide double the voltage of a single unit cell. Inthis configuration, the amplifiers in the unit cells are substantiallyindependent, and are simply given input commands V_(c-in), 180° out ofphase, i.e., +V_(c-in) and −V_(c-in). No modification to the control ofeither unit cell is needed for BTL operation, though a product intendingto support this mode may handle inverting the V_(c-in) input, ratherthan relying on the user to provide both the original and invertedsignal. Inverting of the V_(c-in) signal may be done in the controlelectronics (not shown), or by an additional inverting amplifier (notshown), either under the control of control electronics or directlycontrolled by a physical switch available to the user.

As shown in FIG. 4, two half bridges 100 a and 100 b may be combined asa parallel pair to provide the same voltage and twice the current of asingle unit cell. In this configuration, one outer voltage loop 112 a isconfigured to feed commands, through the first unit's outer voltage loopsummer 114 a and compensator 116 a, to the inner current loops andamplifying stages of both unit cells through a cross-cell connection 202controlled by a switch 204. In this configuration the second outervoltage loop 112 b and its summer 114 b and compensator 116 b are notused—they may be entirely deactivated, or the signal path from thecompensator 116 b to the summer 132 b may be interrupted. Bothhalf-bridge outputs are coupled to a common input of the loudspeaker 120c, with the other input grounded. The current loops 130 a and 130 b,summers 132 a and 132 b, and compensators 134 a and 134 b controlcurrent sharing between the half bridges by stabilizing each at thetarget current, as discussed above. The switch 204 may be controlled invarious ways, including, for example, by control electronics, by passivecircuitry, or by a physical switch available to the user.

As shown in FIG. 5, four half-bridges 100 a, 100 b, 100 c, and 100 d maybe used together to provide a bridged-parallel configuration deliveringdouble the voltage and double the current of a single unit cell to asingle loudspeaker 120 d. Half-bridges 100 a and 100 b are configured asa first parallel pair with a cross-cell connection 202 a and switch 204a, and coupled to a first input of the loudspeaker 120 d. Half-bridges100 c and 100 d are configured as a second parallel pair with across-cell connection 202 b and switch 204 b, and coupled to the secondloudspeaker input. The second pair 100 c/100 d are given an invertedinput signal −V_(c-in) as in the BTL configuration of FIG. 3.

In some embodiments, the control circuitry of each of the half-bridgeunit cells is independent, such that when two cells are used in the BTLor parallel configuration, the other two may be used as independenthalf-bridges, in the same two-cell configuration as the first two cells,or in the other two-cell configuration. In some examples, pairs or allfour of the amplifier stages (modulator and gate drive) are provided ina single integrated circuit package, such as the TDA8932 from NXPSemiconductors, in Eindhoven, The Netherlands, or the TAS5103 from TexasInstruments in Dallas, Tex., while the transistors, control loops, andoutput filters are added to complete the amplifiers and enable theconfigurability described above. In some examples, the control loops andcross-cell connections are included in the amplifier IC.

As shown in FIG. 6, groups of unit cells can be combined, up to thelimits of the power supply, to form highly configurable systems. In FIG.6, a control module 210 is shown coupled to a number of switchesidentified below. Dotted lines show control signal paths while solidlines show audio signal paths. Two sets of four half-bridge unit cellsare shown, numbered 100 a through 100 h. The connections in the secondset 100 e-100 h are identical to those in the first set 100 a-100 d,though they are shown with the switches in different positions. Eightinputs A through H are available, but not all are used. The switchpositions in FIG. 6 are set to show the first two unit cells 100 a and100 b each providing their respective inputs A and B to separateloudspeakers 120 a, unit cells 100 c and 100 d providing input C to asingle loudspeaker 120 b in a BTL configuration, and unit cells 100 e,100 f, 100 g, and 100 h together providing input E to a loudspeaker 120d in a parallel-bridged configuration, with each pair 100 e/100 f and100 g/100 h powering one input of the loudspeaker in parallel.

For the two unit cells 100 a and 100 b being operated as independenthalf-bridges, a first switch 204 a controls the signal path between thetwo unit cells 100 a and 100 b, for providing common current controlwhen operating in parallel, as discussed above. In the example of FIG.6, switch 204 a is open, because unit cells 100 a and 100 b are actingseparately. Another switch 212 controls which signal is input to theunit cell 100 b. For half-bridge operation, as shown, switch 212 couplesthe signal input “B” to the unit cell 100 b.

For BTL operation, an inverter 214 is available to couple an invertedcopy of input signal “A” to the unit cell 100 b, where the switch 212would provide that signal rather than the input “B” used for half-bridgeoperation. This is the case in the second pair of unit cells 100 c and100 d, where the switch 204 b is open, and a switch 216 is coupling aninverter 218 to the input of unit cell 100 d, providing an inverted copyof input “C”. Another inverter 220 and input switch 222 control theinput to unit cell 100 c, for use in parallel-bridged configurations,discussed below with reference to the second set of four unit cells. InFIG. 6, the input switch 222 is coupling the input “C” to the unit cell100 c.

Unit cells 100 e and 100 f are shown configured for parallel operation.In this mode, the input switch 224 on unit cell 100 f does not provideany signal input to the second unit cell, as the closed switch 204 cprovides the current command signal from inside the unit cell 100 e tothe current feedback loop comparator in unit cell 100 f, skipping thevoltage command input and voltage feedback loop comparator of unit cell100 f. In some configurations, the input of unit cell 100 f isdisconnected internally when used in parallel operation, so the inputswitch 224 may remain coupled to one of its inputs. The inverter 226 isavailable to provide an inverted version of input “E” for use in BTLmode. Unit cells 100 g and 100 h are also shown configured for paralleloperation, and in particular, they are configured for use in aparallel-bridged mode with unit cells 100 e and 100 f. Switch 204 dprovides the current command signal to unit cell 100 h from inside unitcell 100 g, while switch 228 is open and inverter 230 is unused. At unitcell 100 g, a switch 232 couples an inverted copy of input “E” from aninverter 234 to the input of unit cell 100 g, so that the two sets ofparallel half-bridges 100 e/100 f and 100 g/100 h receive E and −E,respectively, as inputs.

Although the switches, inverters, and signal sources are shown externalto the control module 210, some or all of the switches and inverters maybe integrated into the control module, and may be configured inhardware, firmware, or software, depending on the technology used. Ifthe switches and inverters are integrated to the control module, theinputs A through H would also pass through the control module. Thecontrol module may be any suitable device, such as a programmedmicroprocessor, an application-specific integrated circuit, or acollection of discrete devices. The control module may also applydigital signal processing to the inputs A through H, in addition to theswitching and inverting used to configure the amplifier topology. Theremay be a separate control module for each set of four unit cells. Insome cases, the control module may be configured to use all eight unitcells in a cascading pattern of bridged and parallel groupings todeliver the entire capacity of the power supply to one loudspeaker(additional connections between amplifiers and/or signal paths would beneeded, though these may all be provided by a suitable control module).

Such a configuration is capable of driving numerous configurations ofloudspeakers. For example, an 4000 W amplifier system containing twogroups of four unit cells, at 500 W each, can drive many differentcombinations of speakers, as shown in table 1 (with the actual wattagedepending on the impedance of the particular loudspeakers used).

Unit 1 Unit 2 Unit 3 Unit 4 Unit 5 Unit 6 Unit 7 Unit 8 500 W 500 W 500W 500 W 500 W 500 W 500 W 500 W 1000 W Parallel or BTL 1000 W Parallelor BTL 500 W 500 W 500 W 500 W 1000 W Parallel or BTL 1000 W Parallel orBTL 1000 W Parallel or BTL 500 W 500 W 2000 W Parallel-Bridged 1000 WParallel or BTL 500 W 500 W 2000 W Parallel-Bridged 1000 W Parallel orBTL 1000 W Parallel or BTL 2000 W Parallel-Bridged 2000 WParallel-Bridged

One or more of these groups of four unit cells are combined with anappropriate power supply, such as that described in co-pending U.S.patent application Ser. No. 12/717,216, titled “Power Supply TransientResponse Improving,” and filed the same day as this application, theentire contents of which are incorporated here by reference. In someexamples, an advantageous feature provided within the power supply is asynchronous output rectifier, which solves the ‘bus pumping’ problemgenerally associated with class D half-bridges, and supports efficientoperation of the highly-configurable amplifier stages described above(the bus-pumping problem is explained for reference below). A schematicrepresentation of this synchronous output rectifier 300 is shown in FIG.7. A primary winding 302 is coupled to switches 304, 306, 308, and 310,and a secondary winding 312 is coupled to switches 314, 316, 318, and320. The windings are separated by an isolation barrier 322 and convertthe +400 V supply voltage to the lower voltage used by the amplifiers,+/−80 V in this example. Such a rectifier powers the amplifier stagesdescribed above with the +80 V and −80 V ports coupled to the +V and −Vrails of the amplifiers.

At the voltage levels typically used in an audio power amplifier, e.g.,+/−80V or higher on the secondary windings, the secondary side switches314-320 in this topology would typically be simple rectifiers. Instead,MOSFETs are used with their intrinsic diodes to provide a synchronousrectifier. This allows power to flow in either direction at any of thethree ports of this power converter (+400 V primary, +80 V secondary,−80 V secondary), providing for full four-quadrant operation, asexplained in FIGS. 8A through 8C.

FIG. 8A shows one half-cycle of normal operation, in which power istransferred from the primary winding 302 to the secondary winding 312.In this cycle, transistors 304 and 310 are on, so that current (shown bydotted arrows) flows from the +400 V rail 322 a to the primary-sideground 324 a by going through the primary winding 302 in a firstdirection, illustrated as downward. This induces current to flow in theopposite direction, illustrated as upward, in the secondary winding 312,due to the direction of the windings. That current flows from the −80 Vrail 330 a to the secondary-side ground 326 and from the secondary-sideground 326 to the +80 V rail 328 a through the transistors 314 and 320.As noted above, the use of MOSFETs provides intrinsic diodes across thesource and drain of the transistors.

FIG. 8B shows the other normal half-cycle, in which the transistors 306and 308 are on, so that current flows from the +400 V rail 322 b to theprimary-side ground 324 b by going through the primary winding 302 inthe opposite direction, i.e., upward in the figure. The induced currentin the secondary winding 312 then goes downward, and through the otherset of secondary-side transistors 316 and 318, to again flow from the−80 V rail 330 b to ground 326 and from ground 326 to the +80 V rail 328b, producing the same net power flow as in FIG. 8A.

FIG. 8C shows a different case, in which energy is being sourced fromthe +80 V rail, and flows into both the −80 V and +400 V rails. Notethat the only difference between this and the case in FIG. 8B is thedirection of current from the +80 V and +400 V rails. At transistors306, 308, and 316, current is flowing in the direction of the intrinsicdiodes. At the +80 V 328 b rail, however, current is flowing against thedirection of transistor 318, so that transistor must be switched on. Ina standard power converter with a simple rectifier between the +80 Vrails and the transformer secondary, energy could not flow in thisdirection. The use of MOSFETs in the secondary allows current flow inthis direction. We have not drawn all possible cases, but it can be seenthat the use of synchronous rectifiers on the secondary side allowsenergy to flow between any port and any other port of this system,allowing four-quadrant operation.

At lower output voltages, when the voltage drop across the rectifiersbecomes a significant source of inefficiency, synchronous rectifiers arein common use today. However, they are not commonly used at the highvoltages of audio power amplifiers, as the efficiency increase is verymarginal, and there are significant technical hurdles to overcome tomake the MOSFETs robust with reverse current flow and parasitic dioderecovery. Using the technique in the system described above allowsenergy flow from reactive loads back into the system to be evenlydistributed between all of the storage capacitance in the system, and italso solves the bus pumping problem associated with half-bridge class Dstages.

Bus pumping, also known as off-side charging or rail pumping, isexplained with reference to FIGS. 9A-9C. As shown in FIG. 9A, ahalf-bridge class D amplifier 400 operates by switching the outputinductor 416 between two voltage rails 402 and 404, labeled here isgeneric +V and −V. The modulator and gate drive are not shown, and thefilter capacitors 422 and 424 are the only part of the power supplyshown. The switching of transistors 412 and 414 occurs at a ratesignificantly higher than the signals being reproduced, so that at anygiven time we can treat the current in the output inductor 416 asrelatively constant. Thus, for the state when current is flowing out ofthe inductor 416 and into the load 420, there are two switching states.

In the first state, as shown in FIG. 9B, the top transistor 412 is on,and current (shown by the dotted arrow) flows from the +V rail 402. Inthis case, energy is flowing from the power supply to the output filterand the load.

In the second state, as shown in FIG. 9C, the bottom transistor 414 ison, and current flows from the −V rail 404. In this case, energy isactually flowing back from the output filter and the load, and into thepower supply. If the power supply filter capacitances 422 and 424 arelarge enough, they can absorb this energy for the duration of the event,but this would require unusually large capacitances.

The expression for average current regenerated into the rail over a halfsine wave is as follows:

$\frac{\sqrt{2\; p}( {{\sqrt{2\; p}\pi\sqrt{r}} + {( {{- 8} + \pi} )v}} )}{4\;\pi\sqrt{r}v}$where ‘p’ is average power out, ‘v’ is rail voltage, and ‘r’ is loadresistance. For example, a 20 Hz sine wave into a 4Ω load, of sufficientamplitude to give 500 W average power output, in an amplifier with ±80 Vrails, will regenerate an average of 3 A over a half cycle. With 10 000μF of capacitance on that rail, the system will experience a bus pumpingof 7.5 V, which is almost 10% of the nominal. By using synchronousrectification, lower capacitances can be used and the system will stillexperience significantly less perturbation. In one example, with theabove-mentioned voltages, a system with only 3500 μF capacitance perchannel per rail experienced significantly less pumping of the bus thanthat just described.

Other implementations are within the scope of the following claims andother claims to which the applicant may be entitled.

What is claimed is:
 1. An audio power amplifier comprising: a first, asecond, a third, and a fourth amplification unit, each amplificationunit comprising a switching voltage amplifier having a command signalinput and an amplified signal output; control electronics providingsignal inputs to each of the amplification units and controlling theamplification units such that: each amplification unit is operable todrive a separate load; the first and second amplification units areoperable in parallel driving a common terminal of a first load, and thethird and fourth amplification units are operable in parallel driving acommon terminal of a second load; the first and second amplificationunits are operable to drive a first bridge-tied-load, and the third andfourth amplification units are operable to drive a secondbridge-tied-load; and all four of the amplification units are operabletogether with the first and second amplification units in paralleldriving a first side of a common bridge-tied-load, and the third andfourth amplification units in parallel driving a second side of thecommon bridge-tied-load.
 2. The audio power amplifier of claim 1,wherein each amplification unit is operable to drive a separate load byeach amplifying separate signals and providing their respectiveamplified signals on their separate output terminals.
 3. The audio poweramplifier of claim 1, wherein: the first and second amplification unitsare operable in parallel driving a common terminal of a first load byeach amplifying a first signal and providing substantially identicalamplified signals on their separate output terminals, which are coupledto the common terminal of the first load; and the third and fourthamplification units are operable in parallel driving a common terminalof a second load by each amplifying a second signal and providingsubstantially identical amplified signals on their separate outputterminals, which are coupled to the common terminal of the second load.4. The audio power amplifier of claim 1, wherein: the first and secondamplification units are operable to drive a first bridge-tied-load byamplifying a first signal in the first amplification unit and amplifyingan inverse of the first signal in the second amplification unit, andproviding their respective amplified signals on their separate outputterminals, which are coupled to separate input terminals of the firstbridge-tied-load; and the third and fourth amplification units areoperable to drive a second bridge-tied-load by amplifying a secondsignal in the third amplification unit and amplifying an inverse of thesecond signal in the fourth amplification unit, and providing theirrespective amplified signals on their separate output terminals, whichare coupled to separate input terminals of the second bridge-tied-load.5. The audio power amplifier of claim 1, wherein all four of theamplification units are operable together by: amplifying a first signalin each of the first and second amplification units, and providingsubstantially identical amplified first signals on separate outputterminals of the first amplification unit and the second amplificationunit; and amplifying an inverse of the first signal in each of the thirdand fourth amplification units, and providing substantially identicalamplified inverted first signals on separate output terminals of thethird amplification unit and the fourth amplification unit, wherein theoutput terminals of the first and second amplification units are coupledto a first input of the common bridge-tied-load, and the outputterminals of the third and fourth amplification units are coupled to asecond input of the common bridge-tied-load.
 6. The audio poweramplifier of claim 1, wherein the switching voltage amplifiers eachcomprise a modulator, a gate driver, a pair of transistors, and a pairof diodes coupled between the source and drain terminals of thetransistors.
 7. The audio power amplifier of claim 6, wherein thetransistors comprise MOSFETs, the diodes being intrinsic to the MOSFETs.8. The audio power amplifier of claim 1, further comprising afour-quadrant power supply having a synchronous output rectifier.
 9. Theaudio power amplifier of claim 1, wherein each amplification unitfurther comprises: an output filter between the amplified signal outputand a load terminal; a current compensator with a current-compensatedcommand signal output coupled to the command signal input of theswitching voltage amplifier; an inner current feedback loop feeding ameasurement of current measured at the output filter back to a summinginput of the current compensator; a voltage compensator with avoltage-compensated command signal output coupled to the summing inputof the current compensator; and an outer voltage feedback loop feedingvoltage at the load terminal back to a summing input of the voltagecompensator.
 10. The audio power amplifier of claim 9, wherein theoutput filter comprises an output inductor and the measured current isthe current through the output inductor.
 11. An audio power amplifiercomprising: a first, a second, a third, and a fourth amplification unit,each amplification unit comprising a switching voltage amplifier havinga command signal input and an amplified signal output; controlelectronics providing signal inputs to each of the amplification unitsand controlling the amplification units such that the amplificationunits are configurable to operate in each of four modes, the four modescomprising: a first mode, wherein each amplification unit drives aseparate load; a second mode, wherein the first and second amplificationunits drive a first bridge-tied-load, and the third and fourthamplification units drive a second bridge-tied-load; a third mode,wherein the first and second amplification units in parallel drive acommon terminal of a first load, and the third and fourth amplificationunits in parallel drive a common terminal of a second load; and a fourthmode, wherein the first and second amplification units in parallel drivea first side of a common bridge-tied load, and the third and fourthamplification units in parallel drive a second side of the commonbridge-tied-load.
 12. The audio power amplifier of claim 11, wherein inthe first mode, each amplification unit drives a separate load by eachamplifying separate signals and providing their respective amplifiedsignals on their separate output terminals.
 13. The audio poweramplifier of claim 11, wherein in the second mode: the first and secondamplification drive a first bridge-tied-load by amplifying a firstsignal in the first amplification unit and amplifying an inverse of thefirst signal in the second amplification unit, and providing theirrespective amplified signals on their separate output terminals, whichare coupled to separate input terminals of the first bridge-tied-load;and the third and fourth amplification units drive a secondbridge-tied-load by amplifying a second signal in the thirdamplification unit and amplifying an inverse of the second signal in thefourth amplification unit, and providing their respective amplifiedsignals on their separate output terminals, which are coupled toseparate input terminals of the second bridge-tied-load.
 14. The audiopower amplifier of claim 11, wherein in the third mode: the first andsecond amplification units in parallel drive a common terminal of afirst load by each amplifying a first signal and providing substantiallyidentical amplified signals on their separate output terminals, whichare coupled to the common terminal of the first load; and the third andfourth amplification units in parallel drive a common terminal of asecond load by each amplifying a second signal and providingsubstantially identical amplified signals on their separate outputterminals, which are coupled to the common terminal of the second load.15. The audio power amplifier of claim 11, wherein all four of theamplification units are operable together by: amplifying a first signalin each of the first and second amplification units, and providingsubstantially identical amplified first signals on separate outputterminals of the first amplification unit and the second amplificationunit; and amplifying an inverse of the first signal in each of the thirdand fourth amplification units, and providing substantially identicalamplified inverted first signals on separate output terminals of thethird amplification unit and the fourth amplification unit, wherein theoutput terminals of the first and second amplification units are coupledto a first input of the common bridge-tied-load, and the outputterminals of the third and fourth amplification units are coupled to asecond input of the common bridge-tied-load.
 16. The audio poweramplifier of claim 11, wherein the switching voltage amplifiers eachcomprise a modulator, a gate driver, a pair of transistors, and a pairof diodes coupled between the source and drain terminals of thetransistors.
 17. The audio power amplifier of claim 16, wherein thetransistors comprise MOSFETs, the diodes being intrinsic to the MOSFETs.18. The audio power amplifier of claim 11, further comprising afour-quadrant power supply having a synchronous output rectifier. 19.The audio power amplifier of claim 11, wherein each amplification unitfurther comprises: an output filter between the amplified signal outputand a load terminal; a current compensator with a current-compensatedcommand signal output coupled to the command signal input of theswitching voltage amplifier; an inner current feedback loop feeding ameasurement of current measured at the output filter back to a summinginput of the current compensator; a voltage compensator with avoltage-compensated command signal output coupled to the summing inputof the current compensator; and an outer voltage feedback loop feedingvoltage at the load terminal back to a summing input of the voltagecompensator.
 20. The audio power amplifier of claim 19, wherein theoutput filter comprises an output inductor and the measured current isthe current through the output inductor.
 21. An audio power amplifiercomprising: a first, a second, a third, and a fourth amplification unit,each amplification unit comprising a switching voltage amplifier havinga command signal input and an amplified signal output; and controlelectronics providing signal inputs to each of the amplification unitsand controlling the amplification units such that the amplificationunits are configurable to drive: four loads, each in a half-bridgeconfiguration, two loads, each in a bridge-tied-load configuration, twoloads, each in a parallel configuration, and one load in aparallel-bridged configuration.
 22. The audio power amplifier of claim21, wherein when four loads are driven, each in a half-bridgeconfiguration, each amplification unit drives a separate load by eachamplifying separate signals and providing their respective amplifiedsignals on their separate output terminals.
 23. The audio poweramplifier of claim 21, wherein when two loads are driven, each in abridge-tied-load configuration: the first and second amplification unitsdrive a first bridge-tied-load by amplifying a first signal in the firstamplification unit and amplifying an inverse of the first signal in thesecond amplification unit, and providing their respective amplifiedsignals on their separate output terminals, which are coupled toseparate input terminals of the first bridge-tied-load; and the thirdand fourth amplification units drive a second bridge-tied-load byamplifying a second signal in the third amplification unit andamplifying an inverse of the second signal in the fourth amplificationunit, and providing their respective amplified signals on their separateoutput terminals, which are coupled to separate input terminals of thesecond bridge-tied-load.
 24. The audio power amplifier of claim 21,wherein when two loads are driven, each in a parallel configuration: thefirst and second amplification units in parallel drive a common terminalof a first load by each amplifying a first signal and providingsubstantially identical amplified signals on their separate outputterminals, which are coupled to the common terminal of the first load;and the third and fourth amplification units in parallel drive a commonterminal of a second load by each amplifying a second signal andproviding substantially identical amplified signals on their separateoutput terminals, which are coupled to the common terminal of the secondload.
 25. The audio power amplifier of claim 21, wherein when one loadis driven in a parallel-bridged configuration, all four of theamplification units are operable together by: amplifying a first signalin each of the first and second amplification units, and providingsubstantially identical amplified first signals on separate outputterminals of the first amplification unit and the second amplificationunit; and amplifying an inverse of the first signal in each of the thirdand fourth amplification units, and providing substantially identicalamplified inverted first signals on separate output terminals of thethird amplification unit and the fourth amplification unit, wherein theoutput terminals of the first and second amplification units are coupledto a first input of a common bridge-tied-load, and the output terminalsof the third and fourth amplification units are coupled to a secondinput of the common bridge-tied-load.
 26. The audio power amplifier ofclaim 21, wherein the switching voltage amplifiers each comprise amodulator, a gate driver, a pair of transistors, and a pair of diodescoupled between the source and drain terminals of the transistors. 27.The audio power amplifier of claim 26, wherein the transistors compriseMOSFETs, the diodes being intrinsic to the MOSFETs.
 28. The audio poweramplifier of claim 21, further comprising a four-quadrant power supplyhaving a synchronous output rectifier.
 29. The audio power amplifier ofclaim 21, wherein each amplification unit further comprises: an outputfilter between the amplified signal output and a load terminal; acurrent compensator with a current-compensated command signal outputcoupled to the command signal input of the switching voltage amplifier;an inner current feedback loop feeding a measurement of current measuredat the output filter back to a summing input of the current compensator;a voltage compensator with a voltage-compensated command signal outputcoupled to the summing input of the current compensator; and an outervoltage feedback loop feeding voltage at the load terminal back to asumming input of the voltage compensator.
 30. The audio power amplifierof claim 29, wherein the output filter comprises an output inductor andthe measured current is the current through the output inductor.